1. Field of the Invention
This invention relates to an image forming apparatus in which sheets are conveyed from a sheet supplying unit along a sheet conveying path and an image is formed thereon, and particularly to an improvement in the timing of supplying sheets from the sheet supplying unit.
2. Description of the Related Art
Conventionally, in a conveyance unit for conveying sheets along a sheet conveying path, when the size of a sheet cannot be detected, the timing of supplying sheets from a sheet supplying unit included in the conveyance unit is determined according to one of the three methods described below.
In a first method, the sheet supply operation is started after the previous sheet has passed through the final process. In accordance with the detection output of a sensor disposed in the vicinity of the place where the final process is conducted, it is determined whether or not the previous sheet has passed through the final process. Alternatively, the time period extending between the beginning of the sheet supply operation and the end of the final process is previously set on the basis of experience, and, when the set time period has elapsed, it is determined that the previous sheet has passed through the final process. In a case where a transfer material is toner, the final process with respect to the conveyance of sheets is a fixing process or a sheet discharge process. According to the first method, the sheet supply operation is conducted one by one for each of sheets irrespective of the size of the sheet to be supplied.
In a second method, the time period is previously measured which is required for a sheet of the maximum size on which an image can be formed by the apparatus passing through a given position. The sheet supply operation is started at the interval of the thus measured time period, irrespective of the size of the sheet to be supplied. Accordingly, when the image formation is carried out on the sheet of the maximum size, the sheet supply operation is sequentially started, but, when the image formation is carried out on the sheets the sizes of which are smaller than the maximum size, the operation of supplying the next sheet is started after a predetermined waiting period has elapsed. In this context, the waiting period is equal to the difference between the measured time period for the sheet of the maximum size and the time period required for the sheets of the smaller sizes passing through the given position.
In a third method, when both the front and rear edges of the previous sheet have been detected by a sensor, the operation of supplying the next sheet is started. In the sheet conveyance according to the third method, the number of sheets conveyed into the sheet conveying path of the body of the apparatus is inversely proportional to the sizes of the sheets. During a given time period, hence, sheets of a small size are conveyed in an increased number, and in contrast those of a large size are conveyed in a reduced number.
Since the sheet supply operation is started irrespective of the size of the sheet to be supplied, the first and second methods have a defect that the sheet conveying operation and the image forming operation which corresponds thereto are slow. Particularly, this defect is remarkable when sheets of a small size are used.
In the third method, the sheet supply operation varies in speed according to the sizes of sheets to be supplied, and therefore, even when sheets of a small size are to be supplied, there appears no defect that the image forming operation is slow. However, the third method suffers from another defect that, when the operation of an apparatus is stopped because of any cause, a number of sheets remain in the sheet conveying path of the body of the apparatus to be removed, and it is cumbersome to remove the remaining sheets and these sheets are wasted.